Is this a Firefox plug-in issue, rather than a Robohelp issue?
I think it is an Acrobat issue. I have seen it and with PDFs not linked to Fm or Rh. Sometimes you can see the issue and sometimes you don't. I also have Nitro PDF and that displays the PDF without problems.
If it is a PDF you create, I guess you could try some different settings or use something else to create it to see if the problem goes away.
See www.grainge.org for RoboHelp and Authoring tips